One of the most important devices for measuring fluid flow velocity is the Pitot tube. Such devices, in various forms, are frequently used in industrial processes for monitoring gases and liquids flowing into pipes or conduits. Another important application of Pitot tubes is the measurement of the air speed of an aircraft and the water speed of a vessel.
A simple configuration of Pitot tube, called Pitot-static tube, is L-shaped and comprises two coaxial tubes. The inner tube has an opening, called stagnation port, which faces the fluid flow and senses the stagnation pressure (or total pressure) of the fluid. The outer tube has pressure sampling holes on its sides to measure the static pressure (or reference pressure). Both the stagnation tube and static tube have tubing connections at the top of the Pitot tube. A manometer is connected to the tubing connections to measure the difference between stagnation and static pressures, that is the dynamic pressure. Velocity of the fluid can then be determined from the dynamic pressure and the known fluid density, using the Bernoulli relation.
The Pitot tubes presently used in aeronautics are not miniaturized, although it would be desirable in the development of smaller unmanned aircraft or drones. Miniature Pitot tubes are however employed in wind tunnels and in water tanks for the measurement of the turbulence of a fluid as well as in liquid helium, both in normal fluid state and the superfluid state. Indeed, the physics of superfluid turbulence requires a measurement of fluid motion on a microscopic length scale, thus calling for an extreme miniaturization of the Pitot tubes.
In the document [“Turbulent velocity spectra in superfluid flows”, Physics of fluid 22, 125102, 2010], J. Salort et al. disclose a stagnation pressure probe operating like a Pitot tube for the measurement of the turbulence in a cryogenic helium flow. Contrary to other types of flow sensor, this probe works both above and below the superfluid transition temperature of helium (Tλ=2.17 K).
FIG. 1 schematically represents the stagnation pressure probe 100 disclosed in the above-mentioned document. The probe 100 comprises a cupronickel outer tube 110 and a conical-shaped nozzle 120, mounted on one end of the outer tube 110. The tip of the nozzle 120 is formed by the end of a cupronickel capillary tube 130 extending through the nozzle 120 in the direction of the flow, designated by the arrow “F”. The sub-millimetric diameter of the capillary tube 130 allows superfluid turbulence to be measured. The nozzle 120 adapts the small diameter of the capillary tube 130 to the larger diameter ϕ of the outer tube 110 (ϕ=3.5 mm), thus reducing the disturbances of the helium flow.
As shown in FIG. 1, a commercial cryogenic pressure transducer 140 is enclosed in the outer tube 110 of the probe. It comprises a diaphragm 141, on which a piezoresistive gauge is disposed (not shown). The diaphragm 141 extends in a plane perpendicular to the axis of the outer tube 110, i.e. perpendicular to the direction of the fluid flow F, and separates a first cavity 142a from a second cavity 142b. The first cavity 142a is connected to the stagnation port of the probe, i.e. the tip of the nozzle 120, through the capillary tube 130. The second cavity 142b is open to the rear of the outer tube 110, where reference pressure is taken via holes 111 made in the outer tube 110. The piezoresistive gauge measures the deformation of the diaphragm 141, which is representative of the difference between the stagnation pressure in the first cavity 142a and the reference pressure in the second cavity 142b. 
The pressure probe 100 of FIG. 1 has problems of robustness, because the (silicon-based) diaphragm 141 needs to be very flexible in order to achieve reasonable sensitivity and may break in extreme conditions, especially for large flow velocity or upon cryogenic cool-down. Furthermore, the volume between the tip of the nozzle 120 and the diaphragm 141 constitutes a dead volume that increases the response time of the probe. As a result, the pressure probe has a limited frequency response.
Finally, the sensibility of the pressure probe 100 is low because the diaphragm 141 has a small area. Yet, increasing the diaphragm area is difficult, since it would increase the diameter ϕ of the outer tube 110 and, consequently, the invasiveness of the probe with respect to the fluid flow. Increasing the diameter of the diaphragm and the outer tube would also increase the dead volume corresponding to the first cavity 6a, further limiting the frequency response of the probe.